Abstract

Iron is an important constituent of the Earth’s silicate mantle. As one of the transition
metal elements, iron can adopt different electronic states (i.e., oxidation, spin states, etc.)
that gives rise to diverse behavior in mantle minerals at corresponding pressure-temperature
conditions. The electronic state of iron can be influenced by important parameters of the
mantle such as the conditions at which mantle minerals and their assemblies were formed
(for example oxygen fugacity), and changes in the electronic state of iron can directly affect
geophysical and geochemical properties of mantle assemblages (such as elasticity, element
partitioning, diffusion, etc.). This PhD thesis focusses on to the evolution and changes of
the electronic state of iron in important mantle minerals and their influence on mineral
properties. In particular, the following problems have been studied:
∙ the effect of Fe3+ in the octahedral site (Y-site) on the sound velocities of garnet solid
solutions,
∙ the iron oxidation state in majoritic garnet inclusions from diamond originating from
the lowermost upper mantle and transition zone and estimation of the corresponding
oxygen fugacities,
∙ systematics of the pressure-induced spin transition of Fe3+ in the oxygen coordination
octahedron and correlations of hyperfine parameters with octahedron geometry,
∙ iron electronic state in bridgmanite synthesized in situ in a diamond anvil cell.
The high-pressure study of skiagite (77 mol. %)-iron-majorite garnet revealed that sound
velocities are significantly lower than the sound velocities of the silicate garnet
end-members, such as grossular, pyrope, Mg-majorite, andradite, and almandine. In addition,
sound velocities of the two garnet end-members with Fe3+ in the Y-site, skiagite and
khoharite, were estimated. The neglect of Fe3+ in the Y-site of the garnet structure may
result in an overestimation of up to 1 % of the sound velocities of the garnet solid solution
that are relevant to the mantle transition zone .
Thirteen garnet inclusions from diamonds of mantle origin were studied using M¨ossbauer
spectroscopy and single-crystal X-ray diffraction. The studied garnet inclusions show
a systematic increase of the iron oxidation state with increase of the formation depth. The
determined oxygen fugacities appear to be higher than the stability field of Fe metal. This implies
that the iron disproportionation reaction (3Fe2+ → 2Fe3+ + Fe0) cannot be responsible
for the high Fe3+ content in the studied mantle garnets, but the hypothesis that carbonate
was the oxidizing agent might be valid.
The comparative study of the spin transition in Fe3+O6 octahedra in FeBO3, Fe2O3,
Fe3(Fe1.766(2)Si0.234(2))(SiO4)3, FeOOH, CaFe2O4 and Ca3Fe2(SiO4)3 showed that the spin
transition of Fe3+ begins within a narrow range of octahedron volumes (8.9–9.3 ˚A3). Taking
into account the compressibility of the Fe3+O6 octahedra, this volume range corresponds to a
45–60 GPa pressure range. It was demonstrated that a simple model of an ideal octahedron
based on crystal field theory predicts transition volumes with reasonable accuracy if the
iron octahedron is not significantly distorted. It was found that, in the case of octahedral
coordination, the center shift of high-spin iron depends linearly on octahedral volume with
the same slope, independent of the oxidation state.
Spin transitions usually lead to an isosymmetric structural transition that can progress
as either supercritical crossover or a first-order phase transition. The position of the critical
point on the phase diagram is determined to a large extent by elastic interactions between
ions in different spin states. Our experimental results suggest a cooperative behavior of iron
ions at room temperature if iron octahedra share common oxygen atoms. As mantle minerals
are solid solutions with relatively low concentrations of iron, the cooperative behavior of iron
ions is unlikely. Therefore, the crossover behavior at iron spin transitions in Earth’s mantle
minerals is more probable, especially taking into account the high mantle temperatures at
spin-transition pressures.
Experiments on the synthesis of bridgmanite in the laser-heated diamond anvil cell revealed
that the Fe3+/ΣFe ratio in bridgmanite depends on the iron oxidation state of the
precursor that was used. We demonstrated that Fe3+ in bridgmanite is formed due to iron
disproportionation in case of synthesis from a reduced precursor at pressures below 60 GPa.
All products of the iron disproportionation reaction, including Fe metal, were identified
in situ.
The Fe3+/ΣFe ratio in bridgmanite synthesized from a reduced precursor at pressures
between 35 and 60 GPa and ∼ 2400 K is about 25 %. In bridgmanite synthesized at 86 GPa
and ∼ 2800 K, Fe3+ adopts the low-spin state and the Fe3+/ΣFe ratio reaches 60 %. However,
despite this high Fe3+ content, we could not detect the presence of Fe metal, which leaves open
the question regarding the origin of the large amount of Fe3+. We argue that the appearance
of the doublet with extremely high quadrupole splitting in M¨ossbauer spectra of bridgmanite
above 30 GPa is related not to the spin transition of Fe2+ in the pseudo-dodecahedral site, but
to a transition between the non-degenerate and Jahn-Teller active electronic terms without
any change in the spin quantum number.